The present invention generally pertains to communication signal modulation and is particularly directed to a novel system that is capable of modulating either analog or digital information onto a carrier signal for transmission over a bandwidth-limited transmission medium.
The more prevalent prior art techniques for modulating carrier signals are frequency modulation (FM), phase modulation (PM) and amplitude modulation (AM).
In conventional FM, information is impressed upon a carrier signal by deviating the frequency of the carrier signal by an amount proportional to the instantaneous amplitude of the modulating signal. Normally, this is done over complete cycles of the deviated carrier signal since, with the most common detection techniques, there is no information in a partial cycle. While FM has no abrupt amplitude changes, it can have abrupt phase changes, and the frequency of the modulating analog information signal must be kept well below the frequency of the carrier signal. In addition, FM requires an externally synchronized clock to recover the signal, since the phase of the deviated carrier carries no information itself. Furthermore, while FM systems do not usually contemplate partial cycle modulation (the usual requirement being a full cycle to define the frequency), if each partial cycle of a FM signal is varied in duration to represent the information, the result can contain a DC component that is not acceptable to most transmission or recording media.
In conventional PM, information is impressed upon a carrier signal by deviating the phase of the carrier signal from the original angle (with no modulating signal) by an amount proportional to the instantaneous amplitude of the modulating signal. PM is generally subject to the same restrictions as FM.
In conventional AM, information is impressed upon a carrier signal by deviated the amplitude of the carrier signal by an amount proportional to the instantaneous amplitude of the modulating signal. Again, the frequency of the modulating analog information signal must be lower than that of the carrier signal to avoid unwanted DC components. Also, AM is subject to error if the transmission or storage medium suffers from amplitude variations. Furthermore, if the amplitude of each half-cycle of a true AM signal is independently varied, the result can contain a DC component that is not acceptable to most transmission or recording media.